Micro electromechanical systems (MEMS) sensors are widely used in consumer and industrial devices. Gyroscopic sensors are one example of type of commonly deployed MEMS sensors. The small size and durability of MEMS gyroscopes enable incorporation of these sensors into mobile electronic devices, automotive and aerospace applications, robotics, cameras, and a wide range of other electronic and electromechanical devices.
A MEMS gyroscope typically includes a vibratory gyroscopic sensor element that oscillates along a first axis at a predetermined frequency. The vibratory sensor element is sometimes referred to as a proof mass and is modeled as a mass that oscillates along an axis to stretch and compress one or more springs. If the MEMS gyroscope experiences rotation, the angular acceleration of the rotation acts upon the MEMS gyroscope to change the position of the proof mass relative to the anchored frame of the MEMS along a sense axis that is perpendicular to the axis of oscillation for the proof mass. Thus, the rotation of the gyroscopic sensor results in a variation between the distance of the vibratory sensor element in the gyroscopic sensor and one or more capacitive sensor electrodes that are positioned in the sensor along the sense axis that is perpendicular to the oscillation axis. As is known in the art, a change in the distance that separates two plates in a capacitor changes the capacitance of the capacitor, and the motion of the vibratory sensor element along the sense axis changes the capacitance between the sensor element and the capacitive sensor electrodes. A sensing circuit detects the changes in capacitance to identify rotation of the gyroscopic sensor as the vibratory member in the gyroscopic sensor moves closer to or farther from the capacitive sensing elements that are positioned in the gyroscopic sensor.
In state of the art gyroscopic sensors, an external power source generates an electrical signal that drives the vibratory sensor element to sustain oscillation of the vibratory sensor element during operation of the gyroscopic sensor. The magnitude of the drive signal is much larger (often on the order of +/−1 V) than the sensed signal (often on the order of microvolts). Due to parasitic coupling between the drive electrodes and the sensing electrodes, the sense electrodes may detect the much larger amplitude drive signal, which is referred to as a “feed-through” signal, in addition to detecting the lower amplitude sensing signal. The feed-through signal is not desirable for detection by the sensing circuit because the magnitude of the feed-through signal is typically much larger than the sensing signal, which makes detection of the much smaller amplitude sensing signal difficult. While a constant level of feed-through can be tolerated in the detection of the sense signal, varying levels of feed-through caused by changes in the drive signal and by environmental factors around the gyroscopic sensor, such as temperature and mechanical stress, make detection of the sense signal difficult.
To reduce or eliminate feed-through, state of the art gyroscopic sensors generate the drive signal for the vibratory gyroscopic sensor element with a square waveform. The square waveform reduces feed-through because the magnitude of the drive signal only changes at the predetermined rising and falling edges of the square waveform. During the comparatively long period of time between the edges, the square wave has a substantially constant level that reduces the effects of feed-through.
While existing gyroscopic sensors are configured to reduce the effects of feed-through, the square wave signal that the drive circuit generates to maintain oscillation of the vibratory gyroscopic sensing element also produces multiple frequency harmonics. In some instances, the harmonics of the square wave signal induce oscillation of the vibratory sensor element at an unintended frequency. For example, a vibratory gyroscopic sensor element that has a resonant frequency of 25 KHz might vibrate at 50 KHz or 75 KHz due to the second and third harmonic of the drive signal. The higher frequencies of oscillation are referred to as parasitic modes and the gyroscopic sensor does not produce accurate output signals when operating in a parasitic mode. While some types of continuously varying drive signals avoid generating the harmonics of the square wave signals, the continuously varying drive signals reintroduce the feed-through noise because the time varying signal does not settle to a constant value in the same manner as the square wave. Consequently, modifications to gyroscopic sensors that improve the rejection of parasitic operating modes while also reducing or eliminating feed-through would be beneficial.